1. Field of the Invention
This invention relates to powder metal parts, and more particularly to a powder metal process which utilizes cryogenic treatment to improve the mechanical properties of powder metal parts.
2. Description of the Prior Art
The mechanical properties of metal parts or components manufactured by conventional powder metallurgical techniques are usually inferior to those fabricated from the same alloys by traditional wrought metallurgy. For structural applications, the most important properties are ultimate tensile strength (UTS), yield strength, elongation and impact strength. Yet these are the properties that tend to suffer from conventional powder metal processing, and are the reason powder metal parts are often not the first choice for high strength, structural applications.
Conventional powder metal parts are typically manufactured by mixing metal powders, pressing in a mold to form a compact of the desired shape, and sintering to arrive at the final product. Secondary operations may be used to improve the mechanical properties. One is heat treatment, which normally involves maintaining the part at an elevated temperature for several hours following a quench into water, oil or a gas. Because of thermally induced stresses, the part may be warmed to a moderate temperature to temper the effect of quenching. A neutral or carburizing atmosphere may be necessary for carrying out this heat treatment.
Cryogenic treatment of metal parts not made by powder metallurgy has been used for many years to harden or increase the wear resistance properties of such parts. For example, U.S. Pat. No. 3,891,477 issued June 24, 1975, to Lance et al. discloses a method of treating cast iron or steel by cryogenic cooling to improve resistance to wear and corrosion. U.S. Pat. No. 3,661,655 issued May 9, 1972, to L. J. Hrusovky describes cryogenic cooling of a spring leaf, cut from a steel blank and formed by taper rolling, to improve fatigue life and stress corrosion resistance. U.S. Pat. No. 4,336,077 issued June 22, 1982, to Leach et al. discloses cryogenically hardening a cast metal alloy insert such as a piston ring. These are but exemplary of many patents describing a variety of cryogenic treatment for metal parts not made by powder metallurgy.
More recently, it has been suggested that cryogenic treatment may be applied to powder metallurgy as an extension of the heat treatment process described earlier. The powder metal part is cryogenically cooled, not quenched, to less than minus 300.degree. F. for several hours as described by Rick Frey in an article entitled "Cryogenic Treatment Improves Properties of Drills and P/M Parts" published in Industrial Heating, September 1983, pages 21-23. The article concludes that the hardness and abrasive wear resistance of heat treated, high carbon powder metal parts can be improved through cryogenic treatment. The parts examined were spurs and gears where such properties are important.
Applicant is not aware of any prior art which suggests that cryogenic treatment can improve the tensile strength of powder metal parts to the point that they can compete with wrought parts for structural uses. Applicant has discovered, however, that certain powder metal alloys, when processed in accordance with the present invention, including cryogenic treatment, will produce parts with tensile strengths of about 200,000 psi or greater.
In conventional powder metallurgy, tensile strengths of about 180,000 psi can be obtained only by heat treatment of certain iron-nickel steels. In the heat treated condition, however, these powder metal parts usually have elongation of less than 0.5% and Unnotched Charpy impact strength of less than 10 ft-lbs. Thus even though high tensile strength is obtained, the elongation and impact strength (toughness) properties suffer. The parts are very brittle and cannot sustain any appreciable stress or deformation in dynamic loading applications.
In contrast, powder metal parts made in accordance with the present invention not only have high tensile strengths of about 200,000 psi, but also have higher elongation (about 3%) and higher Unnotched Charpy impact strength (about 25 ft-lbs) making them much better suited for dynamic loading applications.